python: Replace the image crate with png We weren’t using any of its features anyway, so the png crate is exactly what we need, without the many heavy dependencies of image. https://github.com/image-rs/image-png/pull/670 will eventually make it even faster to build.

cdef struct Vector:
    float x, y, z

cdef Vector sub(Vector vec1, Vector vec2)
cdef Vector cross(Vector vec1, Vector vec2)
cdef float dot(Vector vec1, Vector vec2) nogil
cdef Vector normalize(Vector vec)